In our earlier U.S. Pat. No. 4,762,046 issued on Aug. 9, 1988 we describe a sound reverberator device which is provided in a housing which is detachably securable over a top wall of the sound box rearwardly of the bridge piece whereby to produce sound reverberations when the strings of the guitar are set in vibration. We have now found it desirable to produce a permanent sound reverberator inside the sound box of a string musical instrument, such as a guitar, violin, etc. However, this posed a series of difficult technical problems, such as the manner in which the vibrations from the strings can be transferred to the springs internally of the sound box and how the springs can be stopped from vibrating should it be desirable not to have reverberated sound. Because the springs are disposed internally of the sound box another problem was to devise a manner in which the springs can be placed in pre-tension.